Communicating over a network often involves a variety of tasks. For example, to send content (e.g., a web-page, e-mail, streaming video, etc.) from one device to another, the content is typically divided into portions carried by different packets. An individual packet includes a payload that stores some portion of the content being sent and a header that includes data used in delivering the packet to its destination. By analogy, the packet's payload is much like a letter being mailed while the header stores information (e.g., a network destination address) that appears on the envelope.
Generally, before transmission, a framer maps one or more packets (or packet portions) into a logical organization of bits known as a frame. In addition to packet data, a frame often includes flags (e.g., start and end of frame flags), a frame checksum that enables a receiver to determine whether transmission errors occurred, and so forth. The framer feeds frame bits to one or more devices that generates signals to be carried over a network connection. For example, for an optic signal, the framer feeds a serializer/deserializer (SERDES) and transceiver that generates optic signals representing the digital data of a frame.
Processing a received frame generally proceeds in the reverse of the process described above. That is, a device physically receives signals over a network connection, determines bit values corresponding to the signals, and passes the bits to a framer. The framer identifies frames within the bit stream and can extract packets stored within the frames.
In network terminology, the components described above perform tasks associated with different layers of a network communication “protocol stack.” For example, the bottom layer, often known as the “physical layer”, handles the physical generation and reception of signals. The “link layer” includes tasks associated with framing. Above the physical and link layers are layers that process packets (the “network layer”) and coordinate communication between end-points (the “transport layer”). Above the transport layer sits the “application layer” that processes the content communicated.